Method of manufacture of multiwall paper bags and machine for the practical application of the said method



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In 5% I H :1 u K INVENTOR 3,133,480 5 AND MACHINE ID METHOD 4 Sheets-Sheet 1 (M ToRNEYS I I 1, wi l n 1 A. A. GERARD METHOD OF MANUFACTURE OF MULTIWALL PAPER BAG FOR THE PRACTICAL APPLICATION OF THE SA VIII/I'll!!! fill/Ill)!!! Q'IIIIIIIIIII 71/ a May 19, 1964 Filed June 5, 1962 May 19, 1964 A. A. GERARD 3,133,480

METHOD OF MANUFACTURE OF MULTIWALL PAPER BAGS AND MACHINE FOR THE PRACTICAL APPLICATION OF THE SAID METHOD Filed June 5, 1962 4 Sheets-Sheet 2 INVENTOR ANDREAGERARD May 19, 1964 A. A. GERARD 3,133,480 METHOD OF MANUFACTURE OF MULTIWALL PAPER BAGS AND MACHINE FOR THE PRACTICAL APPLICATION OF THE SA 1962 IDMETHOD Filed June 5,

4 Sheets-Sheet 3 III 1 I INVENTOR A N DR EABE R AR D 5d 54 59 I IIIHHIIIII y 19, 1964 A A. GERARD 3,133,480

METHOD OF MANUFACTURE O F MULTIWALL PAPER BAGS AND MACHINE FOR THE PRACTICAL APPLICATION OF THE SAID METHOD 4 Sheets-Sheet 4 Filed June 5, 1962 INVENTOR ANDREAGERARD Mam ATTORNEYS United States Patent METHOD OF MANUFAETURE 0F MUL'HWALL PAPER BAGS AND MAQHENE FGR THE P6- TlCAL APPLICATEON OF THE SAHD h IETHOD Andr Auguste Gerard, Petit-Quevilly, France, assignor to Les Emballages dc Queviily, Petit-Queviiiy, France, a

society of France Filed June 5, 1962, Ser. No. 2fitl,l% Claims priority, application France .iune 6, 1961 7 Claims. (Cl. 3-35) The present invention has mainly for its object an improved method of manufacture or" paper bags of the type known as multiwall bags, that is to say bags which are composed of a number of superposed sheets of paper and more especially bags of this type comprising an inner Wall of waterproof material such as paper coated or overlaid with a heat-sealed plastic material.

Means have long been sought in the packaging industry to produce bags of this latter type which are made only by sealing and glueing and which oifer at the same time qualities of strength and fluid-tightness, such qualities making them suitable for the bagging of even very heavy products, or products which are sensitive to the action of moisture or of gases, or products in powdered form, paste form or even liquids. It has not been found possible up to the present time, however, to achieve these results by means which readily lend themselves to industrial manufacture in large-scale production.

The present invention provides a remarkably satisfactory solution to this problem while at the same time being suited to the manufacture of multiwall bags of all capacities and especially large capacity bags and indifferently of the types known as fiat bags, expanding bags with tucks down the sides, open-mouth or valvemouth bags, and bags having either shaped or non-shaped bottoms.

To this end, the method in accordance with the invention, in which the bags are manufactured from a continuously formed tube cut into lengths and comprising an inner wall or lining of waterproof and heat-sealing material which is independently closed with a longitudinal sealing joint and which is surrounded by a number of other paper walls sealed longitudinally by glueing, is essentially characterized in that the longitudinal sealing joint of the inner fluid-tight wall is continuously effected over a marginal portion of the width of a hem formed in a manner known per se along one of the longitudinal edges of the'sa-id wall.

When manufacturing bags formed by cutting lengths from a tube which is thus continuously formed, the inner wall of waterproof and heat-sealing material of each bag length is provided at each end thereof with a transverse sealing joint which is intended to ensure the fluid-tightness of the bag. In the practical application of the present invention, the said transverse sealing joint is preferably formed also over a marginal portion only of a fold which is made in one of the transverse edges of the inner wall. Under these conditions, non-sealed reserves of waterproof material are accordingly formed both across the width and along the length of the said inner wall, thereby permitting of subsequent extension of the said wall in both directions, with the result that the sealing joints do not have to withstand the pressure of the product which the bag is required to contain, the said pressure being withstood solely by the walls of paper which surround the inner waterproof wall. As will be understood, there consequently results a remarkable fluid-tightness of the bag.

stated in the foregoing, to all known types of'multiwall paper bags, whether flat bags or bags with side tucks or The arrangements defined above are applicable, as

gussets, bags with non-shaped bottoms or with shaped bottoms, bags with open mouths or valve mouths. In the particular case of flat bags with shaped bottoms and open-mouth or valve-mouth bags, the present invention additionally makes provision for a particular method of forming the bottom as well as certain forms of embodiment of the valve when the said method is applied to the manufacture of such bags as comprise a valve.

In accordance with this method, the starting material employed is a length of tube which is formed in the manner which has been explained above and which is additionally provided with longitudinal scoring lines intended to give to the finished bag a parallelepipedal shape. The inner waterproof and heat-sealing wall of the said-tube length is joined to the outer walls of paper along a transverse glueing line wln'ch is set back with respect to the end of the tube length by a distance equal to one-half the thickness of the filled bag increased by one-half the Width of lap of the outer walls on the finally formed bottom, whereupon air is introduced in the interior of the tube length thus formed prior to transversely sealing the corresponding end of each bottom to be formed, following which, after sealing of the said end, the air which has thus been introduced is forced and compressed towards the sealed end so as to cause the assembly of outer walls to open out and thus make the inner wall acces sible so as to permit this latter to be straightened and given its final shape by flattening, the previous compression operation being then interrupted in order to permit the air to escape through the other end of the tube length 'or through the valve, depending on whether the said bag is of the open-mouth type or valve type.

When the bag to be made is of the valve type, the valve is formed by a sleeve of the same material as that of the inner wall and inserted laterally in the longitudinal sealing joint of the tube at the time of forming this latter. The said sleeve can be either set back with respect to one of the ends of each tube length corresponding to a bag and at a distance from the said end which is equal to one-half the thickness of the bottom increased by one half the width of lap of the outer walls on the finally formed bottom, namely exactly at the end of the tube length.

The present invention further comprises certain special devices for the-purpose of continuously forming, in accordance with the method which has been defined above, the starting tube from which are cut the lengths intended for making bags, as well as for the purpose of effecting the closure of the bottoms of the said bags in the case of fiat bags of the shaped bottom type. The characteristic features of the said devices and other particular features of the invention will be brought out by the complementary description which is given below, reference being made to the accompanying drawings which are given by way of example only without any limitation being implied, and in which:

FIG. 1 is a partial diagrammatic cross-section of one mode of execution of a machine which is used for the continuous forming of the improved starting tube in accordance with the invention;

FIG. 2 is an enlarged transverse cross-section of the inner Waterproof wall of the tube such as it appears after having been sealed in the machine in accordance with FIG. 1;

FIG. 3 is a transverse cross-section of the machine taken along the line III-III of FIG. 1;

FIG. 4 is an enlarged transverse cross-section of the tube which is formed on the said machine;

FIG. 5a and FIG. 5b are two portions, which are joined together along the line z-z, of a diagrammatic view in elevation of a machine which is intended to perform the bottom-forming operation;

FIGS. 6 to 12 are diagrammatic longitudinal crosssections of a bag at various stages of the bottom-forming operation;

FIGS. 13 and 14 are plan views of the bottom of the bag such as it appears respectively after the inner wall has been given its final shape, and after complete formation;

FIG. 15 is a further enlarged cross-section of the completed bottom;

FIG. 16 is a transverse cross-section of the machine which is used for the formation of the starting tube shown at the stage at which a valve is being fitted to the tube;

FIGS. 17 and 18 are partial plan views of a tube length which is intended to form a bag, illustrating respectively two modes of arrangement of the valve.

In the example which has been illustrated diagrammatically in FIG. 1, the machine which is intended for the continuous forming of the tube from which the bags are to be constructed is designed to form a tube consisting of four walls, namely, an inner wall of waterproof material such as a paper which is coated with a heatsealing plastic material, and three paper walls which successively surround the inner wall.

The reference 1 designates the band of coated paper or the like which is intended to form the inner waterproof wall and which is supplied from the roll 2, and the references 1', 1", 1" designate the bands of paper which are intended to form the outer walls which are supplied from the rolls 2', 2 and 2". The said bands are displaced longitudinally at a pre-determined speed of forward movement by known driving means not shown in the drawings and placed at that end of the machine which is opposite to the rolls 2, 2', 2" and 2".

The band 1 is first of all fed into a device 3 of conventional type which folds back one of the longitudinal edges of the said band so as to form a hem 4 (as shown in FIG. 2) which has a width L of two centimetres, for example, and which is required to subsequently permit the longitudinal sealing of the inner wall by the application of one coated face against the other coated face.

The bands of paper 1, 1", 1", after having passed on their side over a glueing roller 5 which glues in a suitable manner one of the longitudinal edges of the said bands, join the band 1 beneath a member 6 having the shape of a ski-board tip and also of conventional type, which is intended to initiate the shaping of the tube.

While the edges of the bands 1', 1", 1" are held apart by symmetrically arranged metallic fingers 7 (as shown in FIG. 3), the edges of the band 1 pass inside a pre-heater 8 which brings the plastic layer to the desired temperature for sealing and are then drawn together and superposed above a metallic plate or batten 9 in such manner that the longitudinal joint which is thus formed is flattened beneath the sealing rollers 10. In conformity with one of the characteristic features of the present invention, and as can be clearly seen in FIG. 3, the arrangement and thickness of the sealing rollers 10 are such that the seal 11 is formed over only a fraction of the width of the hem 4 which has been previously formed along one of the edges of the band 1 and close to the outer edge of the said hem. The width l (as shown in FIG. 2) of that portion which has thus been sealed is equal, for example, to one-half the width L of the hem, namely one centimeter. As has been explained above, the remaining width of one centimeter which thus remains free, that is to say non-sealed, permits of a certain extension of the inner wall 1 in the transverse direction, with the result that the joint 11 is not required to be subjected to the pressure of the product which is contained in the bag, the said pressure being solely withstood by the outer walls of paper.

After the operation which has just been described, the edges of the paper walls 1', 1", 1" are in turn drawn together, superposed and glued in the usual and known manner. The said paper walls then completely surround the wall 1 around the batten 9 and the tube is accordingly formed in continuous manner. The transverse cross section of the said tube is shown diagrammatically in FIG. 4, the glued joints of the walls 1, I", 1" being designated in the figure by the references 12', 12", 12".

The tube which is obtained in the manner which has just been set forth is constructed in the form of a flat sleeve which can directly be employed for the manufacture of so-called flat bags. When it is required to construct so-called expanding bags, that is to say bags having side tucks or gussets which open out as and when the bags are filled, means of known types (which have not been illustrated in the drawings) are provided at an appropriate point of the machine so as to continuously form these tucks on both sides of the bag.

In all cases, the finished tube is cut off as it passes out of the machine into sections of suitable length which correspond to the intended size of the bags, by means of a conventional device which has not been illustrated in the drawings.

In the case of fiat bags with shaped bottoms, the bagforming operation is preferably carried out in the manner which is described below.

FIG. 6 illustrates in diagrammatic longitudinal crosssection the tube length which is intended to form the bag such as it appears at the input end of the machine for forming the bottoms which are illustrated in FIGS. 5a and 5b. In order to simplify the drawing, there have only been shown the inner plasticized wall 1 and the adjacent outer paper wall 1". The said tube length, which is constructed in a flattened form, is in principle marked on each face thereof with two longitudinal scoring lines such as 51, 51" (as shown in FIGS. 17 and 18), which are located symmetrically at a suitable distance from each lateral edge of the said tube length and which are intended to permit this latter to assume a parallelepipedal shape at the time of filling of the bag. The inner wall 1 has been provided, as described above, with a hem or longitudinal fold by means of which the sealing along the length of the said inner wall has been effected with a coated face applied against a coated face, while the sealing joint 11 only extends over a marginal portion of the said hem or fold so as to provide a nonsealed reserve by virtue of which the expansion of the inner plasticized wall is greater than that of the outer walls of paper.

At the input end of the bottom-forming machine, the tube length is received transversely between suckers 52, 53 (as shown in FIGS. 5a and 7) which are intended to ensure the introduction of a certain quantity of air between the walls of the tube. The bottom suckers 52 are stationarily fixed in the vertical direction and maintain the underface of the tube against the bed of the machine (which has not been illustrated) while the top suckers 53 are movable and lift the upper face so as to permit the introduction of air.

The tube length, accompanied by the suckers 52, 53, is then taken between endless belts 54 (as shown in FIGS. 5a and 8) which are intended to convey the said tube length and which apply appropriate pressure against the ends of this latter in order to ensure that the air which has just been introduced inside the said tube length is maintained imprisoned therein. On each side of the said endless belts are located electric preheaters 55 which bring to a suitable temperature for the sealing operation either the two ends of the tube if it is required to construct a valve-type bag or one end only if it is required to manufacture an open-mouth bag.

At the output end of the preheaters 55 are mounted pairs of sealing rollers 56 (as shown in FIGS. 5a and 9) which weld transversely at 57 the two ends of the tube in the case of a valve-type bag or only one of these ends in the case of an open-mouth bag.

Pairs of scoring rollers 58 (as shown in FIGS. 5a and 10) are placed next in succession to the sealing rollers and impress, at appropriate distances away from each sealing joint 57 or from one of said joints alone depending on whether the bag made is of the valve type or open-mouth type, scoring lines which are designed to permit the folding of the bottom or bottoms in accordance with the process which will be described hereunder.

When the necessary scoring lines have been marked, the tube passes beneath a compressor belt 59 (as shown in FIGS, a and 11), the action of which has the result of driving towards the sealed end of the tube the air which is imprisoned in this latter. The said end is accordingly inflated and deformed along the tucking lines 60', 61)" which have previously been marked, the said deformation being facilitated by means of a plate 61 which maintains the tube applied against the bed of the machine.

During this operation, the lips which are formed by the outer walls 1" open out, as can be seen in FIG. ll while the inner wall 1 is retained by transverse lines of glue 62 which are laid at the moment of forming the tube at those places where the tucking lines 60', 60 are later to be marked, that is to say at a distance from the end of the tube length which is equal to one-half the thickness of the bottom increased by one-half the width of lap of the 'outer walls on the finally formed bottom. This opening of the lips of the outer walls permits of the insertion of a pallet 63 which has a slanting edge 63a of appropriate outline and to which is imparted a rotary movement about a vertical axis xx. While performing the said rotary movement, the said pallet 63 thrusts back and straightens the inner wall 1 (as shown in FIG. 12) at the same time as the top portion of the outer Walls 1". At this moment, and in order to permit this action, the belt 59 has ceased to function, with the result that the air which is imprisoned is allowed to escape outside the tube, either through the open mouth or through the opening provided for the valve. There can be seen at 20 the marginal reserve which is provided, as has been stated above, between the transverse sealing joint 57 and the scoring line 60" whichis formed for this purpose.

FIG. 13 represents a plan view of the bottom such as it appears at the end of the preceding operation. The inner wall 1 has then assumed its final shape while the ends of the outer Walls 1" are formed flat on each side of the inner wall, the flattening thereof having been completed by means of a pulley 64 (as shown in FIG. 5a) which is mounted next in succession to the pallet 63 and the rim of which has an appropriate shape for this purpose.

The bottoming operation is completed by the conventional method. A glueing disc 65 (as shown inFIGS. 5a and 5b) deposits glue on both those portions of the outer walls 1 which are intended to be folded back, this folding operation as well as the glueing of the portions which are folded back being carried out by means of a system 66 of known type comprising a mandrel and a set of metallic linkrods. FIG. 14 represents a plan view of the bottom which is finally formed. A strengthening band (which has not been illustrated in the drawings) can finally be glued on to the folded-back portions of the outer walls and in a manner which is also known, by means of a drum 67 placed at the output end of the machine, as shown in FIG. 511.

FIG. 15 shows in cross-section and on a larger scale, the bottom of the bag which is thus formed. At the time of filling of the bag, the scoring lines 60', 60" be come the transverse ridges of the parallelepiped, the longitudinal ridges of which are formed by the longitu dinal scoring lines 51', 51" which have been previously referred-to. The mechanical strength of the bag is en sured by glueing together the front and rear faces of all the outer walls 1". The fluid-tightness of the bag is ensured as a result of the heat-sealing of the inner Wall and in such manner that, at any given moment, the volume of this latter cannot be less than that of the outer walls which contain the said inner wall. The strength of the heat-sealed portions is in turn ensured by virtue of the fact that they are not at any time subjected to the pressure applied to the bag, this pressure being solely withstood by the combined assembly of outer walls.

In the case of an open-mouth bag, the operations which have been described above are employed only for the purpose of forming a single bottom. To permit of closure of the bag after filling, the forming of the top closure has been previously initiated by the provision of scoring lines disposed in exactly the same fashion as the scoring lines of the bottom closure. It is merely necessary to seal the mouth by flattening this latter in an appropriate manner so as to prevent any overthickness, then to fold back along the scoring lines by hand, thereby permitting the bag to be closed finally either by stapling, sewing or glueing. The two ends of the bag then have the same appearance. The bag has the shape of a regular parallelepiped, and is thereby endowed with the maximum capacity as well as the greatest ease of storage by stacking. When the bag constructed is of the valve type, the bottom and top closures are formed in the manner which has been described in the foregoing, but the tube length which is intended to form the bag is fitted at one end, at the time of the tube-forming operation, with a sleeve of coated paper of the same kind as the paper which goes to make the inner wall 1 of the tube. As can be seen in FIG. 16, which shows a transverse cross-section of the machine employed for the continuous forming of the starting tube as illustrated in FIG. 3, and as described above, the said sleeve which is designated in the figure by the reference 68 is disposed at right angles to the direction of movement of the paper and is inserted, at intervals corresponding to the size of the bags to be manufactured, between the longitudinal edges of the inner wall 1 and in such manner as to be gripped and sealed inside the longitudinal sealing joint 11. The sleeve 68 is in principle engaged inside the inner wall 1 over a length which is substantially equal to the width of the sealing joint 11, namely a width, for example, of one centimeter, and extends beyond the said wall by a length which can be equal to multiples of approximately three centimeters corresponding to the number of non-coated outer walls with which the said tube is provided. The position of the said sleeve in thedirection of the length of the tube can give rise to alternative forms.

In accordance with a first alternative form as shown in FIG. 17, the sleeve 68 is sealed at a distance d from the end of the tube length corresponding to one bag which is equal to one-half the thickness of the formed bottom increased by one-half the width of lap of the outer walls, that is to say, substantially at the level of the top scoring line 60". The said sleeve is accordingly held at the same time inside the zone of longitudinal glueing of all the outer Walls of the tube.

In accordance with a second alternative form as shown in FIG. 18, the sleeve 68 is placed flush with the tube end and is accordingly sealed only in the inner wall of the tube. The open end thereof accordingly extends from the bottom of the bag at the time of forming of the said bottom.

In both these two alternatives, the open end of the valve formed by the sleeve 68 is sealed after filling of the bag, then tucked inside this latter.

What I claim is:

1. In a process for the continuous forming of a tube for the manufacture of multiwall paper bags from an inner web of waterproof and heat-sealing material which is independently folded into a tube and closed by a longitudinal sealing joint and a plurality of paper webs which surround said inner web and which are individually closed longitudinally thereof by glueing, the step which consists in continuously doubling back one of the longitudinal marginal portions of said inner wall during the tube forming operation, overlapping the other longitudinal marginal portion on said first marginal portion forming a hem and sealing the marginal portion only of the width of said hem.

2. In a process for the continuous forming of a tube for the manufacture of multiwall paper bags from an inner web of waterproof and heat-sealing material which is independently folded into a tube and closed by a longitudinal sealing joint and a plurality of paper webs which surround said inner web and which are individually closed longitudinally thereof by glueing, the step which consists in continuously doubling back one of the longitudinal marginal portions of said inner wall during the tube forming operation, at the same time overlapping the other longitudinal marginal portion on said first marginal portion forming a hem and sealing the marginal portion only of the width of said hem, cutting said tube into sections each of a suitable length to form a bag, and providing said inner web of each of said tube sections and at each end of said section which corresponds to a bag bottom, with a transverse sealing joint formed over a marginal portion only of one of the transverse edges of said inner web.

3. In a process as claimed in claim 2, the steps which consist in providing on said tube during the continuous forming thereof longitudinal scoring lines arranged to form a bag of a parallelepipedal shape, joining said inner web of each tube length to said outer paper webs at each end of said tube length corresponding to a bottom of the bag and by means of a transverse glueing line which is spaced with respect to said end of the tube length a distance which is equal to one-half the thickness of the filled bag increased by one-half the width of the lap of the outer walls on the finally formed bottom, introducing air into the interior of said tube length prior to transversely sealing the end which corresponds to each bottom to be formed, and then after the transverse sealing of said end, causing the air which has thus been introduced to be forced back and compressed towards the sealed end opening out the end portion of said outer walls, folding back the end portion of said inner web along one of said glue lines and one side of said outer paper webs, compressing said inner web folded end portion to its permanent shape, interrupting said compression permitting the air to escape through the other end of said tube length and sealing the end portions of said outer paper webs while overlapping beyond said glue lines.

4. In a process as claimed in claim 2, including inserting a tubular sleeve of waterproof and heat sealing material laterally of and inside said longitudinal sealing joint of said inner web at the time of the tube-forming operation and at a distance from one of the ends of each tube length corresponding to a bag which equal to onehalf the thickness of the bottom increased by one-half the width of the lap of the outer webs on the finally formed bottom providing a valve for the bags.

5. In a process as claimed in claim 2, including inserting a tubular sleeve of waterproof and heat sealing material laterally of and inside said longitudinal sealing joint of said inner web at the time of the tube forming operation and at one of the ends of each tube length corresponding to one bag.

6. A machine for the continuous forming of concentric tubes for the manufacture of multiwall paper bags provided with an inner tubular wall of waterproof and heat-sealing material having a longitudinal sealing joint and a plurality of'paper walls surrounding said inner wall and having overlapping edges sealed by glue, said machine comprising a series of rollers for a web of waterproof and heat-sealing material for forming said inner wall, a plurality of paper webs forming the walls surrounding said inner wall, means for feeding said webs in superposed relationship, means for applying glue to the edges of said paper webs, a member having the shape of a ski-tip positioned for slideably receiving said webs and capable of initiating the forming of said tube, means for folding back a marginal portion of said first web and overlapping the other marginal edge forming a hem, means for locally preheating said hem preparatory to effecting the longitudinal sealing thereof, and a series of roller dies capable of pressing a marginal portion of said hem for effecting the sealing thereof and means for effecting the longitudinal glueing of the paper webs together.

7. Machine for bottoming multiwall paper bags starting from bag lengths cut from a tube fitted with an inner tubular wall of waterproof and heat-sealing material having a longitudinal sealing joint and surrounded by a plurality of paper walls having overlapping edges sealed by glueing, said machine comprising in combination with a horizontal table for slideably receiving said tube lengths, suction members positioned at the input end of said table and being capable of lifting the combined assembly of walls forming the upper face of each tube length for admitting air into said tube length, two pairs of conveyor belts positioned for receiving said tube length from said suction members and which are capable of applying pressure against the ends margins of said tube length which maintains the air in the interior of said tube length, preheaters disposed in parallel relation with and adjacent to at least one of said pairs of conveyor belts, sets of sealing rollers and scoring rollers positioned next in succession to at least one of said pairs of conveyor belts for receiving said tube length therefrom and capable of successively transversely sealing at least one of the two ends of said tube length and then marking transverse scoring lines on said tube length to permit the folding of the corresponding bag bottom, a compressor belt positioned next in succession to said rollers for receiving said tube length and capable of pressing the air contained in said tube length'towards the sealed end of said tube length, a rotatably driven pallet positioned next in succession to said compressor belt and having an edge of appropriate configuration positioned to enter between the outer walls of paper of said tube length and fold back the inner wall at the same time as the upper outer wall, and means for flattening the bottom portions of said tube length forming the bottom thereof, applying glue to the ends of the outer walls and folding back the ends of said outer walls along said scoring lines enclosing said inner wall.

References Cited in the file of this patent UNITED STATES PATENTS 2,169,936 Wagner Aug. 15, 1939 2,228,647 Waters Jan. 14, 1941 2,641,167 Gramegna June 9, 1953 2,648,263 Richens Aug. 11, 1953 2,895,387 Robinson et al. July 21, 1959 2,913,966 Stageberg Nov. 24, 1959 3,025,768 Kessler Mar. 20, 1962 3,040,967 Klein June 26, 1962 3,043,199 Niemeyer July 10, 1962 

1. IN A PROCESS FOR THE CONTINUOUS FORMING OF A TUBE FOR THE MANUFACTURE OF MULTIWALL PAPER BAGS FROM AN INNER WEB OF WATERPROOF AND HEAT-SEALING MATERIAL WHICH IS INDEPENDENTLY FOLDED INTO A TUBE AND CLOSED BY A LONGITUDINAL SEALING JOINT AND A PLURALITY OF PAPER WEBS WHICH SURROUND SAID INENR WEB AND WHICH ARE INDIVIDUALLY CLOSED LONGITUDINALLY THEREOF BY GLUEING, THE STEP WHICH CONSISTS IN CONTINUOUSLY DOUBLING BACK ONE OF THE ONGITUDINAL MARGAINAL PORTIONS OF SAID INNER WALL DURING THE TUBE FORMING OPERATION, OVERLAPPING THE OTHER LONGITUDINAL MARGINAL PORTIN ON SAID DIRST MARGINAL PORTION FORMING A HEM AND SEALING THE MARGINAL PORTION ONLY OF THE WIDTH OF SAID HEM. 